The following represents a glossary of terms used within the specification. The reader is referred to these definitions when interpreting the meaning of any description herein.    1) {dot over (V)}E: minute ventilation (the total volume of gas breathed in and out of the lung per minute). PETCO2: end tidal PCO2 (the partial pressure of CO2 at end exhalation);    2) SGF: source gas flow (the flow of gas into a breathing circuit, in liters/min);    3) {dot over (V)}A: alveolar ventilation (that ventilation that results in gas exchange between the pulmonary capillaries and the air spaces (alveoli) of the lung), expressed in liters/min. It is also defined as the total ventilation minus the ventilation of the anatomic dead space ({dot over (V)}E−{dot over (V)}Dan);    4) {dot over (V)}O2: The O2 consumed in liters per minute;    5) {dot over (V)}CO2: The CO2 produced in liters per minute;    6) FSx: fractional concentration of a gas x in source gas (gas entering a breathing circuit);    7) FIx: fractional concentration of gas x in inspired gas (gas entering the patient's lungs);    8) FEx: the fractional concentration of gas x in end expired gas;    9) CBC (conditional breathing circuit): a breathing circuit in which only exhaled gas, and no source gas, exits from the circuit, as would be the case with a circle circuit with a low flow of gas entering the circuit, or a Magill circuit in which a) SGF is ≦0.7×{dot over (V)}E; b) the common breathing tube volume (see FIG. 1) is greater than or equal to the sum of [(SGF×expiratory time)+the anatomical dead space]; c) the volume of the breathing bag is greater than [the largest expected tidal volume−(SGF×inspiratory time)].Introduction
The measurement of uptake and elimination of gases via the lungs plays an important role in medicine. Oxygen consumption ({dot over (V)}O2) and CO2 production ({dot over (V)}CO2) are two important parameters indicating cardio-respiratory fitness of athletes. {dot over (V)}O2 and {dot over (V)}CO2 are also used as important indicators of the efficacy of therapeutic intervention in critically ill patients. The ability to impose a transient change in {dot over (V)}O2 and {dot over (V)}CO2 allows one to calculate such important physiological parameters as cardiac output and functional residual capacity. For the most part, anesthesia is induced and maintained by gases taken up by and eliminated from the body via the lungs. Accurate measurement and control of uptake and elimination of anesthetic gases would improve the control of anesthetic depth and thereby the effectiveness and efficiency of the use of anesthetic gases. Accurate control of uptake and elimination of therapeutic gases would allow more controlled dosing when these gases are used as therapies for illness. Accurate control of uptake and elimination of inert gases via the lung can be used for various diagnostic and research purposes.
Present Art
Measuring Gas Flux
Measuring total gas flux requires the measurement of gas volumes for discrete periods of time and multiplying these volumes by the concentration of the gas in the volume.
Volume Measurements
Measurement of exhaled gas volumes is very cumbersome in clinical or research settings. One method requires timed collections of exhaled gas in bags and then measuring the volume of the bags. Inhaled volumes are even more awkward to measure continuously as the volumes enter the lung and one must measure the volume of lung expansion or the volume depleted from a previously known volume. This cannot be done breath-by-breath. These measurements are usually simplified by measuring flow continuously at the mouth and integrating the flow electronically with respect to time to obtain “continuous” measures of volume. Each type of flow measuring device has inherent problems leading to inaccuracy of calculation of volume (see below).
Flux Measurements
To measure the flux of a particular constituent (gas x) of the total gas that enters or exits the lung is more complex. The concentration of x sampled at the mouth during breathing changes between inhalation and exhalation as well as continuously during each ventilatory phase. Therefore, to measure the flux of gas x, the concentration of gas x must be measured continuously with a rapidly responding gas analyzer, and the average concentrations over short intervals must be multiplied by the volume changes over those same intervals. This requires synchronization of flow-volume signals and gas concentration signals, then multiplying the values and continuously summing them. A number of devices on the market such as the Vmax (Sensormedics, Yorba Linda Calif.), Medical Graphics CPOX/D, (Medical Graphics Corporation, St. Paul, Minn.) and NICO (Novametrix, Wallingford, Conn.) measure the fluxes of CO2 and/or O2 at the mouth using this method. The same principles apply to measuring the flux of other gases if appropriate gas sensors are used.
Measuring Gas Fluxes During Anesthesia
a) Understanding the Anesthetic Circuits
One circuit used for anesthesia is the Magill circuit with the Mapleson A configuration illustrated in FIG. 1 (“Magill circuit”). The patient breathes through the patient port (30). During inhalation, gas is drawn from the source gas port (33) and the gas reservoir bag (34) along the common breathing tube (32). Expiration is divided into two phases. The first phase lasts from the beginning of exhalation until the filling of the gas reservoir bag (34). During this first phase of exhalation, expired gas proceeds down the common breathing tube (32) with gas from the anatomical dead space preceding gas from the alveoli. Expired gas displaces gas in the breathing tube (32) into the gas reservoir bag. During his phase of exhalation the source gas is also directed into the gas reservoir bag. The second phase of exhalation is from the time of filling of the gas reservoir bag (34) until the beginning of inhalation. During this second phase of exhalation, the expired gas exits through the one way pressure relief valve (31) that has an opening pressure of about 2 cm H2O and the source gas proceeds along the common breathing tube (32) displacing gas before it and forcing it out of the pressure relief valve such that the last exhaled (alveolar gas) exits the valve first.
Kain and Nunn (Kain M. L., Nunn J. F. Anesthesiology. 29: 964-974, 1968) determined the minimum source gas flow required to prevent rebreathing in anesthetized patients breathing through the circuit by sequentially decreasing the source gas flow until minute ventilation and end tidal PCO2 increased. It is generally accepted that the source gas flow needed to prevent rebreathing of alveolar gas is 70% of the minute ventilation ({dot over (V)}E) (Understanding Anesthesia Equipment by Dorsch J. A., and Dorsch S. E., Williams & Wilkins Co. 1975, pg. 169). The 30% savings in source gas is due to the rebreathing of the anatomical dead space gas which does not undergo gas exchange in the alveoli and therefore retains the same composition as source gas.
The 30% savings in source gas flow with the Magill breathing circuit represents the maximum efficiency available for source gas without the use of a CO2 absorber. As the cost of anesthesia varies directly with the flow of source gas, circuits with CO2 absorbers, the most popular being the “circle circuit” depicted in FIG. 2, allows a marked reduction in source gas flow (SGF) without causing a rise in end tidal PCO2. The balance of {dot over (V)}E is provided by rebreathing of previously exhaled gases and the CO2 absorber (6) prevents the build-up of CO2 in the circuit and the patient. As not all of the delivered anesthetic is extracted during a breath, exhaled gas has a considerable concentration of anesthetic that can be re-supplied to the patient when rebreathed. The circle circuit contains a patient port (1), and an expiratory limb (2) leading to a one way expiratory valve (3). Distal to the valve there is a flexible gas reservoir (4), a pressure relief valve (5) where excess expired gas is vented, and a container for CO2-absorbing crystals (6). When the patient inhales, he draws fresh gas entering the fresh gas inlet (7) and makes up the balance of inspired gas by drawing gas from the gas reservoir through the CO2-absorber. The source gas and the previously exhaled gas join to flow through the one-way inspiratory valve (8) to the patient through the inspiratory limb (9). When the patient exhales, gas passes down the expiratory limb of the circuit (2), past the expiratory valve (3), and enters the flexible gas reservoir (4). When the gas reservoir fills to capacity, pressure in the circuit increases and the pressure relief valve (5) opens, releasing gas to atmosphere during the remainder of exhalation. During exhalation, fresh gas entering the circuit (7) is displaced back into the CO2 absorber (6). This fresh gas enters the inspiratory limb (9) and is made available to the patient on subsequent breath(s).
The depletion of O2 and anesthetic from the circuit is prevented by re-supplying both gases through the fresh gas inlet (7). The anesthesiologist can control the total flow of gas as well as the concentrations of all its constituent components such as oxygen, nitrous oxide and anesthetic agent. The minimum gas flow into the circuit is that needed to replace the oxygen consumed and anesthetic absorbed by the body. The CO2 absorbers extract only CO2, allowing other gases to pass through unchanged.
b) Calculation of Uptake or Elimination of Gases with Rebreathing Circuits (Present Art):
When a subject breathes via a Magill, circle, or Fisher isocapnia (rebreathing and non-rebreathing) circuit, and the SGF entering the circuit is equal to or greater than {dot over (V)}E, the circuit acts like a nonrebreathing circuit, i.e., inspired concentration of gas x is that of the SGP, i.e., FSx. When SGF is less than {dot over (V)}E, inspired gas is composed of both SGF and previously exhaled gas in the Magill, circle and Fisher rebreathing isocapnia circuit; and composed of SGF and reserve gas in the Fisher non-rebreathing isocapnia circuit. As a result, the concentration of x varies throughout inspiration in a complex way depending on {dot over (V)}E, pattern of breathing, and SGF. To measure the inspired volume of x, inspiration must be broken up into small intervals during which the total volume must be multiplied by the average concentration of x; the resulting discrete volumes of x must be summed for the duration of inspiration. Similarly, to calculate the expired volume of x, continuous measurements of expired flows and expired concentrations of x are required. The net uptake or elimination of x over a given time is the algebraic sum of the inhaled and exhaled volumes of x during that time.